It is true that sensors have become better and IBIS is very good as well. But there is another reason for still wanting faster lenses:

- Typically a lens is at its best when stopped down at least 1 stop but in most cases 2 stops.

That's true as a purely descriptive empirical generalization, although if we are talking FF lenses rather than MFT lenses, you'd have to add another stop or two. With such lenses, at least if they are pretty fast, it is not unusual that you have to stop down three or even four stops before the optimum is reached.

But why is it that you have to stop down to reach optimal performance? Is it because it is impossible to design an f/4-lens that is just as good wide open at f/4 as an f/2-lens stopped down two stops? I think not. Rather, I think it is because it is easier to keep optical aberrations at bay at f/4 than at f/2.

Personally, I'd much rather have f/2-lenses that peak at f/4 than f/4-lenses that peak wide open even if the performance from f/4 on is just the same. But there is no doubt in my mind that the f/4-lenses we are talking about could be built.

Sure. This could trivially be done. You could restrict the aperture of the PL25 (and probably quite a number of M43 primes) to no wider than f4 and voila you have an f4 lens that peaks (and peaks very well I might add) wide open

Precisely. Though I would think that a 25/4 matching the 25/1.4 at f/4 could be accomplished within somewhat smaller dimensions.